1. Field of the Invention
This invention relates to an organic electroluminescent (hereinafter abbreviated "EL") device that utilizes electroluminescence of an organic thin film.
2. Description of the Related Art
Conventional EL devices currently prevailing are as those of an alternating-current drive type provided with a highly resistant insulating layer between electrodes. They are roughly grouped into powder EL devices and inorganic thin-film EL devices. The powder EL devices have a laminated structure comprised of an insulating layer formed on an aluminum foil backed electrode in a thickness of several tens of .mu.m. The laminated structure is formed by coating a composition prepared by dispersing a powder with a high dielectric constant such as barium titanate in a resin binder such as cyanoethylpolyvinyl alcohol or cyanoethyl cellulose, an electroluminescent layer containing a zinc sulfide type powder phosphor dispersed in a resin binder, and a transparent electrode formed on a polyethylene terephthalate or glass substrate. Laminated structures of this type can provide surface-emitting devices having a large area and a thickness of 1 mm or less, and are suited to, e.g., back lighting for liquid crystal display. But on the other hand, these laminated structures have the disadvantages that they tend to cause a decrease in luminescence compared with inorganic thin-film types and have so insufficient a threshold value in the applied voltage-luminance characteristics that they have the problem of crosstalk and are not suited to X-Y matrix drive display.
As for the inorganic thin-film EL devices, they have a multilayered structure which includes of a transparent electrode substrate comprising a glass plate coated with indium-tin oxide (hereinafter "ITO") or the like, an yttrium oxide or the like dielectric thin-film layer formed thereon as an insulating layer by sputtering or the like approximately in a thickness of thousands of .ANG., a ZnS, ZnSe, SrS or CaS type phosphor thin film successively deposited thereon by electron beam deposition, sputtering or the like approximately in a thickness of thousands of .ANG., and further successively deposited thereon another dielectric thin-film layer and a back electrode made of aluminum or the like. Here, the layer thickness between electrodes is 1 to 2 .mu.m or less. Such thin-film EL devices have a long lifetime and yet can perform highly precise display, and are suited to their use in display for portable computers. They, however, have the disadvantages that they can not be easily designed for full-color display because of a difficulty in the development of blue electroluminescent materials or white electroluminescent materials having a high luminance and a long lifetime, and are expensive.
In either type of the EL devices, there is another disadvantage that a high alternating voltage of 100 V or more must be applied in order to obtain a sufficient luminance. Accordingly, when, for example, an EL device is light-emitted using batteries, a boosting transformer is required. Hence, even when the EL device itself is of a thin type of 1 mm or less, it is difficult to make smaller the thickness of the whole equipment into which the device is incorporated. Also when the X-Y matrix drive is used, there is the problem that the drive circuit becomes expensive since special ICs for high voltages are required.
Now, in recent years, research has been aiming at low-voltage direct-current drive EL devices that require no boosting transformers. As an outcome thereof, organic EL devices are proposed [Japanese Patent Application Laid-open Nos. 57-51781, 59-194393, 63-264692, 63-295695 and 1-292291; U.S. Pat. Nos. 4,356,429, 4,539,507, 4,769,292 and 4,720,432; Japanese Journal of Applied Physics, 25(9),773(1986); Applied Physics Lett., 51(12),913(1987); Journal of Applied Physics, 65(9),3610(1989); etc.]
These organic EL devices commonly have a multilayered structure comprised of a substrate and successively provided thereon an anode, a hole injecting and transporting layer (hereinafter abbreviated "HITL"), an organic electroluminescent layer (hereinafter abbreviated "OEL") and a cathode, and are fabricated in the following way:
On a transparent insulating substrate such as a glass sheet or resin film, a transparent and conductive anode comprising a compound oxide of indium with tin (ITO) is formed by deposition or sputtering.
Next, on the electrode, an HITL comprising copper phthalocyanine (hereinafter "CuPc"), poly(3-methylthiophene) or a tetraphenyldiamine represented by the formula (1), (2) or (3) shown below is formed by vacuum-vapor deposition or the like in a thickness of about 1 .mu.m or less in the form of a single layer or multilayer comprised of a plurality of said materials. ##STR1## 1,1-bis (4-di-p-tolylaminophenyl)cyclohexane (melting point: 181.4.degree. C.-182.4.degree. C.) ##STR2## N,N,N',N'-tetra-p-tolyl-1,1'-biphenyl-4,4'-diamine (melting point: 120.degree. C.) ##STR3## N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (melting point: 159.degree. C.-163.degree. C.; glass transition point: 67.degree. C., measured by DSC using nitrogen at a rate of temperature rise of 10.degree. C./min; hereinafter "TPD").
Next, on the HITL, an organic phosphor such as tetraphenyl butadiene, anthracene, perylene, coronene, a 12-phthaloperinone derivative, tris(8-quinolinol)aluminum (hereinafter "Alq.sub.3 ") or europium tris(tenolyltrifluoroacetonate)phenanthroline is deposited, or its dispersion in a resin binder is coated, to form the OEL in a thickness of about 1.0 .mu.m or less.
For the purpose of increasing electroluminescent efficiency of the OEL or changing electroluminescent colors, it is also known to dope the Alq.sub.3 electroluminescent layer with about 0.5 to 2 mol % of coumarin-6 (a green electroluminescent coloring matter), 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4-H-pyran (a reddish yellow electroluminescent coloring matter) or the like.
As a final step, on the OEL, a single metal such as Mg, In or Al, or an Mg-Ag alloy (atomic ratio: 10:1), an Mg-Cu alloy, an Mg-Sn alloy, an Mg-In alloy, an Ag-Eu alloy or the like is deposited in a thickness of about 200 nm to form the cathode.
Into the organic EL devices fabricated in the manner described above, holes and electrons are injected by applying a low direct voltage of 20 to 30 V or less setting the transparent electrode side as the anode, so that light is emitted as a result of their recombination to obtain a luminance of about 1,000 cd/m.sup.2.
As other forms of the organic EL devices as described above, proposed are double heterojunction type devices of a triple-layer structure provided with an electron injecting and transporting layer (hereinafter abbreviated "EITL") between an OEL and a cathode, or HITL emitting type electroluminescent devices of a double-layer structure comprised of a hole injecting, transporting and emitting layer and an EITL [Applied Physics Lett., 57(6),531(1990)]. The devices of the triple-layer structure are comprised of an ITO electrode and successively deposited thereon an HITL comprising TPD, an OEL comprising 1-[4-N,N-bis(p-methoxyphenyl)aminostyryl] naphthalene (hereinafter "NSD"), an EITL comprising 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (hereinafter "BPBD"), and a cathode comprising an Mg-Ag alloy. HITL emitting type electroluminescent devices of the double-layer structure have a structure in which the TPD layer is removed from the devices of the above triple-layer structure and the NSD layer serves as both the HITL and the OEL.
As still other forms of organic EL devices, devices of a single-layer structure that can be produced at a low cost are proposed [Applied Physics, Vol. 61, No. 10, page 1044 (1992)]. Devices of this type are comprised of an ITO anode and successively coated thereon a film formed in a thickness of about 100 nm by spin coating or the like of a hole transporting poly(N-vinylcarbazole) solution comprising a mixture of about 1% by weight of coumarin-6 as a electroluminescent material and 20 to 60% by weight of BPBD as an electron transporting material, and then a cathode comprising an Mg-Ag alloy.
Into the organic EL devices fabricated in the manner described above, holes and electrons are injected by applying a low direct voltage of 20 to 30 V or less setting the transparent electrode side as the anode, so that light is emitted as a result of their recombination to obtain a luminance of about 1,000 cd/m.sup.2.
As described above, in the organic EL devices, various methods of layer construction are proposed in order to make organic phosphors emit light, but they are common one another on the point that holes are efficiently injected from the anode into the OEL containing an organic phosphor and electrons are injected from the cathode to make the phosphor emit light. In principle, it can be expected that, with the construction in the order of an anode, an electron-blockable HITL, an OEL, a hole-blockable EITL and a cathode, the carrier confinement effect is utilized to increase the density of electrons and holes in the OEL to thereby increase the rate of recombination, so that an EL device with a high electroluminescent efficiency can be obtained.
However, at present, no well effective electron-blocking, hole injecting and transporting material has been obtained. Accordingly, we consider that the most important goal toward the practical utilization of organic EL devices is to improve durability of the electron-blockable HITL and the cathode.
Incidentally, preferable requirements for cathode materials used in organic EL devices can be said to be as follows:
(1) They have a good adhesion to organic thin films. PA1 (2) They do not tend to be oxidized and are stable. PA1 (3) They have a low work function so that electrons can be readily injected at an energy level of the lowest unoccupied molecular orbital (hereinafter "LUMO") of organic thin-film materials.
The cathode material Mg-Ag alloy (atomic ratio: 10:1; work function: about 3.8 eV) hitherto most used is a material in which Ag is added to improve the poor adhesion to organic thin films that is attributable to Mg having a low work function (work function: about 3.6 eV). However, this material has the problem that it will more likely corrode metal films up to their insides in the moist air than an Mg single material because of formation of a local cell due to the addition of Ag.
The organic EL devices have been also involved in the problem that no satisfactory method of sealing has been hitherto developed because of their weakness to heat at the time of heat sealing or to adhesives containing an organic solvent. Hence, the devices are stored or driven in vacuum or in an inert atmosphere such as dry Ar gas, and it has been sought to develop cathode materials having a low work function and a stability and to develop sealing techniques for achieving stable drive in the air.
The energy level of LUMO (lowest unoccupied molecular orbital) of Alq.sub.3 hitherto known as a typical electron transporting electroluminescent material capable of obtaining the highest luminance is about 3.1 eV as a value determined by subtracting an optical energy gap (2.75 eV) from a value of the work function measured in the atmosphere by the photoemission method using a surface analyzer AC-1, manufactured by Riken Keiki K.K. The energy level is about 2.7 eV in the case of the BPBD used as an EITL. Now, as cathode materials for efficiently injecting electrons into these materials to obtain an organic EL device having a high luminance of 10,000 cd/m.sup.2 or more, expectations can be placed on alkali metals such as Li (work function: 2.9 eV), Na (ditto: 2.75 eV) and K (ditto: 2.15 eV), having a work function smaller than 3.1 eV and a high Fermi level. There, however, is the problem that they are extremely likely to be oxidized and are unstable and hence can not be used as cathode materials.
Another conventional hole injecting and transporting material CuPc is heat-resistant and highly durable and has a high efficiency for the hole injection from ITO. It, however, has a low energy level of LUMO and hence has a low electron blocking performance at the interface of the electroluminescent layer. It has also a large absorption in the wavelength region of visible rays, and also is crystalline to make deposited film surfaces uneven. Hence, devices in which only the CuPc is used as an organic hole injecting and transporting material have had the problem that their efficiency for withdrawing electroluminescence is low and an electrical short of the device tend to occur.
Moreover, although the compounds represented by the formulas (1) to (3) have a higher energy level of LUMO than ordinary electroluminescent materials to have a high electron blocking performance at the interface of the electroluminescent layer, are amorphous to enable formation of deposited films with smooth surfaces and have no absorption at the visible wavelength region, they have had the problem that they have a low glass transition temperature and hence may be mixed with the OEL as a result of heat generation during the fabrication process of devices or the driving of devices, or the films may become crystalline with the lapse of time to make their surfaces uneven. For example, when a TPD [formula (3)] layer and an Alq.sub.3 layer are successively deposited on a glass substrate in the form of thin films of 50 nm thick each, there is the problem that both the layers become mixed at a temperature of about 95.degree. C.
Another HITL is proposed in which the CuPc and any of the compounds represented by the formulas (1) to (3) are deposited in bilayer to reduce the thickness of the CuPc layer to half so that it can be endowed with abilities of electroluminescent light transmission, hole injection and electron blocking to a certain extent. This, however, has an insufficient heat resistance.
There are additional problems that an HITL comprised of only low-molecular weight compounds has a low mechanical strength of the film and a device having an organic layer formed by only deposition of low-molecular weight compounds tends to cause a short at the edge portion, the part having a thin step coverage, of an ITO etching pattern.
In relation thereto, according to Television Society Technical Report, Vol. 16, No. 2, page 47 (1992), a TPD single layer deposited film is used as the HITL and an Alq.sub.3 deposited film to which quinacridone (hereinafter "Qd") has been added is used as the electroluminescent layer to achieve a maximum luminance of 68,000 cd/m.sup.2 (right before melt fracture). This device had an initial luminance of as low as 275 cd/m.sup.2 in its drive at a constant current of 4 mA/cm.sup.2 and had a luminance half-life of 130 hours. In general, organic EL devices have the problem that increasing luminance by increasing current density results in an increase in the speed at which the electrical resistance of the device rises and in an acceleration of the rate of deterioration, and hence it has been difficult to obtain devices having achieved both the high luminance and the long lifetime.